1. Field of the Invention
The present invention relates to batteries, and, in particular, to alkaline primary cells comprising a zinc gel as the anode material, an aqueous alkaline electrolyte, a separator, and a cathode material containing manganese dioxide.
2. Description of the Related Art
The cathodes used in alkaline primary cells customarily comprise manganese dioxide, graphite, and a binder. Additionally, electrolyte, surfactants, and further additives may be present. The manganese dioxide is used, particularly in the form deposited electrochemically (EMD), as the active cathode material.
Synthetic graphite of high purity or alternatively expanded graphite, prepared from natural graphites, in the form of a powder having a typical particle size of, e.g., 10-50 .mu.m in the case of synthetic graphites and a particle size of 1-20 .mu.Mm in the case of expanded graphites, are added to the cathode material as electroconductive material. Graphite has the function, when uniformly dispersed within the cathode compression-molded electrode as a conductive skeletal first-order matrix, of ensuring the electric charge transfer within the cathode. Customarily, the graphite content is from 7 to 10% by weight when synthetic graphites are used. If expanded graphites are used, it is possible, employing special blending techniques, to reduce the graphite content to about 5% by weight, an improvement in the discharge characteristics of the cathode being achieved at the same time.
In many cases, the cathode is composed of cathode rings inserted into the cell jar. The mechanical strength required of these cathode compacts is brought about by a binder. Modern fabrication plants for alkaline primary cells run at very high speed. Thus it is possible, for example, to achieve a production rate of 1000 cells of size LR6 per minute. These so-called "high-speed lines" pose certain minimum requirements with respect to the mechanical strength of the cathode rings which are conveyed in feeder sections and holding plates.
A drawback of most binders is that they require a volume that is not available for active material. Furthermore, many binders are hydrophobic and impede electrolyte uptake of the cathode during the cell fabrication process. This has adverse effects on cell performance.
Typical binders are powdered plastics from the group of the polyethylenes (PE), polypropylenes (PP), polyethylene terephthalates (PET), polytetrafluoroethylene (PTFE), polyacrylates (PA), polybutadienes (PB), and block polymers or copolymers of the abovementioned compounds. The introduction of binders in the form of aqueous dispersions is also known (e.g., PTFE or PE dispersions), the added water likewise having some of the characteristics of a binder with respect to the cathode compact.
The cathode material further comprises additions of alkaline electrolytes, preferably aqueous potassium hydroxide in concentrations of from 10 to 55%. Binary electrolytes such as KOH/NaOH or KOH/LiOH and ternary electrolytes such as KOH/NaOH/LiOH can be used.
An electrolyte is meant to fill the pores of the cathode to achieve ion conductivity in the cathode. A number of positive characteristics are thus achieved. Thus, the use of binders can be entirely or partially dispensed with, since alkalis likewise have some of the characteristics of a binder with respect to the cathode compact. A suitable choice of the amount of electrolyte allows an optimum porosity of the cathode to be set, with the result that the diaphragm resistance of the cathode compact is minimized. A reduced diaphragm resistance in turn distinctly improves the performance of the complete cells. A cathode comprising, e.g., 6% by weight of a 50% strength KOH electrolyte improves the discharge characteristics of the cells fabricated therewith, compared with a cathode containing much less or no electrolyte. Moreover, it is possible, by means of a high concentration of the electrolyte, to distinctly reduce post-storage contact resistance from the cell jar to the cathode ring.
The fabrication of a cathode with a high electrolyte content in the cathode material does, however, have drawbacks with respect to the process, compared with a dry cathode mix. The fabrication of the cathode compact customarily makes use of so-called carousel-type compression molds. These carousel-type compression molds as a rule are made of special steel alloys that are subject to significantly increased wear as the electrolyte content in the cathode formation increases.
The addition of surfactants to the cathode material improves electrolyte uptake of the cathode. The surfactant is customarily added in very low concentrations of, for example, 1-100 ppm, based on the cathode weight, and can be added to the cathode mix both homogeneously and applied to the graphite component in a preliminary step, to reduce the hydrophobic properties of the graphite.
Customary surface-active substances can be liquid or solid and may be of the nonionic, anionic, or cationic type. Thus, for example, aliphatic fluorine compounds, aromatic and aliphatic phosphonic acids, or polyethylene glycols are suitable.
A drawback of such surfactants is occasionally observed, however, in that these, owing to their high molecular mobility, reach the zinc electrode as the counter electrode and there, with certain discharge modes (e.g., pulsed discharge), cause the voltage level to be lowered.
Further additives used include, inter alia, titanium compounds.
U.S. Pat. No. 5,342,712 proposes anatase TiO.sub.2 as an additive to the cathode. The addition of from 0.1 to 2% by weight of the titanium dioxide modification anatase to the cathode material of alkali primary cells is claimed to enable an increase in the period of use by 15% with higher currents (3.9 ohm discharge).
What still remains desirable, however, is a dry-component cathode, i.e., without added electrolyte or with less added electrolyte, which cathode nevertheless in the assembled cell has the advantages of a pasted cathode. Another objective is to extend the service life of carousel-type compression molds for cathode fabrication, which results in cost savings in the production process.
It is an object of the present invention to provide a solution for the abovementioned problems.
Further aspects and advantages of this invention will become apparent from the detailed description which follows.